I’m not sure if there is a real connection between dino strength and Code 19 difficulty.
This morning I had a Code 19 for Erliphosaurus, one of the strongest dinos in my park. First I did not want to accept it, but as the reward was only 5 SDNA suprisingly, I started it and it was relatively easy to complete. I had 5 SDNA Codes before with weaker dinos which were a lot harder.

The number of dinosaurs in the paddock also affects the outcome. The more dinos, the more DNA available for reward (and the harder the Code 19). Some rares with only two dinos, I get max 25 DNA. Other rares with six or seven dinos in the paddock, I can get up to 75 DNA. I don’t know the exact pattern, but there’s some correlation with the number of creatures in the paddock.